1. Field of the Invention
The present invention relates to an optical pickup capable of recording/reproducing information by irradiating a recording medium with light.
2. Description of the Related Art
Recording mediums used in an information recording/reproducing device tend to employ an optical recording system, not a magnetic recording system. The optical recording system has a larger recording capacity and is excellent in convenience of control when recording/reproducing is performed. As a recording medium used for the optical recording system, there are provided a compact disk (abbreviated to CD), a digital versatile disk (abbreviated to DVD) and the like. DVD uses light, having a wavelength shorter than light used in recording/reproducing CD, in recording/reproducing so as to increase a recording capacity.
An optical pickup is used in recording/reproducing information onto and from a recording medium such as a CD or DVD. FIG. 6 is a diagram simply showing the configuration of an optical pickup 1 according to the related art, and FIG. 7 is a view schematically showing the configuration of a hologram element 4 provided in the optical pickup 1. The optical pickup 1 includes a light source 2 emitting light, a grating 3, a hologram element 4, a collimator lens 5, an objective lens 6, and a light receiving unit 7 serving as provided with light receiving elements. The optical pickup 1 records/reproduce information onto and from a disk 8 serving as a recording medium.
A semiconductor laser for emitting a laser beam, or the like is used as for the light source 2. The grating 3 splits light emitted from the light source 2 into three beams including a main beam 11 and first and second sub-beams 12 and 13. The grating 3 has such a structure in which periodical irregularities are formed thereon and diffracts the emitted light so as to generate a plurality of beams. When the irregularities formed on the grating 3 have a simple and periodic rectangular shape, diffracted light components to be generated are zero-order diffracted light (the main beam 11), +first-order diffracted light (the first sub-beam 12), and −first-order diffracted light (the second sub-beam 13). The main beam 11 is a main light flux for obtaining information recorded on the recording medium 8, and the sub-beams 12 and 13 are used in controlling a focused position of the main beam 11. Light split by the grating 3 passes through the hologram element 4. Then, the light passes through the collimator lens 5 so as to become substantially parallel light and is introduced to the objective lens 6.
The objective lens 6 focuses the light emitted from the light source 11 into a recording layer for recording information of the disk 8. The objective lens 6 is provided to be displaced in a direction perpendicular to an optical axis 14 of emitted light, which is introduced to the objective lens 6, within a movable range including a neutral position where the optical axis of the objective lens 6 and the optical axis 14 of the emitted light form the light source 2 are on the same axis. The objective lens 6 is displaced in the direction perpendicular to the optical axis 14 of the emitted light, thereby changing a focused position of the emitted light with respect to the disk 8.
The main beam 11 and the first and second sub-beams 12 and 13 are focused on the disk 8 by the objective lens 6. After passing through the objective lens 6, the main beam 11 and the first and second sub-beams 12 and 13 reflected by the disk 8 pass through the collimator lens 5 so as to be introduced to the hologram element 4.
The hologram element 4, which is provided between the light receiving unit 7 and the objective lens 6, is a light splitting unit having a hologram pattern which includes a plurality of splitting sections for splitting the reflected light from the disk 8 into a plurality of light components.
The hologram pattern of the hologram element 4 includes a splitting section 15 for tracking and a splitting section 16 for focus. The splitting section 15 for tracking splits light used for obtaining track position information which is information on a focused position of emitted light with respect to the disk 8 in the direction perpendicular to the optical axis 14 of the emitted light. The splitting section 16 for focus splits light for obtaining focus position information which is information on a focused position of emitted light in the direction perpendicular to the recording layer of the disk 8.
The hologram pattern is divided into the splitting section 15 for tracking and the splitting section 16 for focus, which are commonly formed in a semi-circular shape, by a first dividing line 17. The first dividing line 17 is parallel to a radial direction (an X direction of FIGS. 6 and 7) of the disk 8 which is mounted on an information recording/reproducing device and is recording or reproducing information. The splitting section 15 for tracking is further divided into two parts by a second dividing line 18 parallel to a tangential direction (a Y direction of FIGS. 6 and 7) with respect to tracks formed on the disk 8, so that two splitting sections for tracking 19 and 20 are formed. Hereinafter, the splitting section 16 for focus is abbreviated to a first region 16, the splitting section 19 for tracking is abbreviated to a second region 19, and the splitting section 20 for tracking is abbreviated to a third region 20.
Here, the X direction as the radial direction and the Y direction as the tangential direction are commonly used in the entire specification. Note that the radial direction of the disk, that is, the recording medium 8 represents a direction along one radial line connecting a point at an intersection between an optical axis of light emitted by the optical pickup apparatus 1 and a recording surface of the recording medium 8, with a center of the recording medium 8. Further, the radial direction is perpendicular to the tangential direction in the recording surface of the recording medium 8.
Depending on a position of the objective lens 6, a position of emitted light spot 21 entering the hologram element 4 changes. For example, when the objective lens 6 is in the neutral position, the main beam 11 reflected from the disk 8 enters the hologram element 4 such that the optical axis of the main beam 11 passes through the center of the hologram pattern. At this time, the main beam 11 and the first and second sub-beams 12 and 13 enter the second region 19 and the third region 20 such that the same proportion of light is delivered to each region. Meanwhile, when the objective lens 6 is in a position deviated from the neutral position in the radial (X) direction, the optical axis of the main beam 11 reflected from the disk 8 is displaced along the first dividing line 17 extending in the X direction. At this time, the main beam 11 reflected by the disk 8 enters the hologram element in such a biased state so that a larger amount of light is delivered to the second region 19 or the third region 20 of the hologram pattern.
FIG. 8 is a top view showing the simplified configuration of the light receiving unit 7. The light receiving unit 7 is a light detecting unit provided with a plurality (eight in the present embodiment) of light receiving elements formed of, for example, photodiodes. The respective light receiving elements are formed in a substantially rectangular shape and are arranged in the Y direction such that the longitudinal directions thereof are positioned in the X direction. The light receiving unit 7 includes a first light receiving section 7A composed of first and second light receiving elements 7a, 7b, a second light receiving section 7B composed of third to fifth light receiving elements 7d, 7g, and 7h, and a third light receiving section 7C composed of sixth to eighth light receiving elements 7c, 7e, and 7f. 
The light, which is reflected by the disk 8 and then enters the first region 16 of the hologram element 4, is diffracted to be introduced into the first light receiving section 7A (the first and second light receiving elements 7a and 7b) for detecting a focus error signal (abbreviated to FES) On the basis of the result of light received by the first light receiving section 7A, an FES is generated.
Among the light beams, which are reflected by the disk 8 and then enter the second region 19 of the hologram element 14, the main beam 11 is introduced to the third light receiving element 7d of the second light receiving section 7B, and the first and second sub-beams 12 and 13 are respectively introduced to the fifth and fourth light receiving elements 7h and 7g of the second light receiving section 7B. Among the light beams, which are reflected by the disk 8 and then enter the third region 20 of the hologram element 4, the main beam 11 is introduced to the sixth light receiving element 7c of the third light receiving section 7C, and the first and second sub-beams 12 and 13 are respectively introduced to the eighth and seventh elements 7f and 7e of the third light receiving section 7C.
On the basis of signals detected by the respective light receiving elements corresponding to the second region 19 and signals detected by the respective light receiving elements corresponding to the third region 20, a tracking error signal (abbreviated to TES), which indicates the displacement in a direction perpendicular to the optical axis of emitted light of the objective lens 6, is detected. Accordingly, a positional deviation of the objective lens 6 from the neutral position in the radial (X) direction is detected by the TES signal.
As for the disk 8 serving as a recording medium, there is provided a CD or DVD as described above. As a disk material of DVD, the same polycarbonate as that of CD is used. However, CD is formed of single plate with a thickness of 1.2 mm, and DVD has such a structure that two disks with a thickness of 0.6 mm are bonded to each other. In DVD having such a structure, light with a short wavelength is used to increase a recording capacity. Further, recording can be performed on two layers, that is, both surfaces, which makes it possible to implement a further increase in recording capacity.
In the case of a disk having a two-layer structure, however, the following problem occurs. FIG. 9 is a view for schematically illustrating reflected light in the case of the disk having two recording layers. The disk 8 of FIG. 9 has a first recording layer 8a as an upper layer and a second recording layer 8b as a lower layer. In the optical pickup 1, when the light from the light source 2 is focused on the first recording layer 8a, most of the light is reflected by the first recording layer 8a so as to become reflected light 22. However, a part of the light transmits through the first recording layer 8a into the second recording layer 8b and is then reflected in the second recording layer 8b. 
As compared with the first recording layer 8a, the second recording layer 8b is in a position more separated from the objective lens 6. Accordingly, the reflected light 22 from the second recording layer 8b is reflected in a position 24 separated in comparison with a focal position of the objective lens 6. In a state where the beam radius of reflected light becomes small by the objective lens 6 and the collimator lens 5, the reflected light 22 enters the hologram element 4. Further, when the reflected light entering the hologram element 4 is diffracted by the hologram element 4, the size of the light spot becomes large on the light receiving unit 7. Therefore, the reflected light can enter light receiving elements other than predetermined light receiving elements.
When the objective lens 6 is in the neutral position, an output signal indicated by a TES on the basis of the reflected light 22 from the first recording layer 8a becomes 0. However, an output value indicated by a TES on the basis of the reflected light 23 from the second recording layer 8b does not become 0, because the first sub-beam 12 enters the fourth light receiving element 7g of the second light receiving section 7B. That is, an output value by the light receiving elements receiving the respective sub-beams does not become 0.
FIG. 10 is a view showing a state where reflected light enters the hologram element 4 when the objective lens 6 is in a position deviated from the neutral position. When the objective lens 6 is deviated from the neutral position, the reflected light 23 from the second recording layer 8b enters any one of the second and third regions 19 and 20 of the hologram pattern, as shown in FIG. 10. In the present embodiment, the reflected light 23 enters only the second region 19.
When the reflected light 23 from the second recording layer 8b enters only the second region 19, the reflected light 23 enters only the second light receiving section 7B corresponding to the second region 19, but does not enter the third region 7C corresponding to the third region 20. When the reflected light 23 from the second recording layer 8b enters only the second region 19 so as to enter only the second light receiving section 7B corresponding to the second region 19, an output value by the light receiving element of the second light receiving section 7B becomes constant even though the objective lens 6 is displaced. Therefore, an offset is generated in an output value indicated by a TES.
In the disk having two recording layers as described above, when the hologram element 4 shown in FIG. 7 is used and information of one recording layer is obtained, the reflected light from the other recording layer exerts an effect. Therefore, it is impossible to accurately calculate a displaced position of the objective lens 6 with respect to the neutral position in the radial (X) direction.
An apparatus in which the shape of a hologram pattern formed in a hologram element is corrected has been proposed in order to solve the problem (refer to Japanese Unexamined Patent Publication JP-A 2004-303296). FIG. 11 is a top view showing the configuration of a hologram element 30 proposed in an optical pickup in the related art. The hologram element 30 includes first to third regions 31 to 33. The first region 31 forms a splitting section for focus, and the second and third regions 32 and 33 forms a splitting section for tracking. Although the hologram element 30 is similar to the hologram element 4 shown in FIG. 7, the hologram element 30 has a different feature in that the disposition of the respective regions and the shapes thereof are different from those of the hologram element 4.
The second and third regions 32 and 33 serving as the splitting section for tracking are formed so as to be disposed in a region excluding an axis neighboring portion 36 of the hologram element 30 which coincides with an optical axis 35 of reflected light 34 introduced to the hologram element 30 when the objective lens 6 is in the neutral position. The axis neighboring portion 36 is formed in a semi-circular shape centered on the optical axis 35.
In other words, the hologram pattern of the hologram element 30, which is formed in a substantially circular shape, is divided into the first region 31 serving as the splitting section for focus and the remaining splitting section for tracking by a first dividing line 37 which is parallel to the radial (X) direction and has a semi-circular curved line portion formed in the center thereof. The remaining splitting section for tracking is divided into the second and third regions 32 and 33 by a second dividing line 38 parallel to the tangential direction. The first region 31 is formed to have the semi-circular axis neighboring portion 36 protruding toward the second and third regions 32 and 33 more than a virtual dividing line 39 connecting both ends of the first dividing line 37 in the semi-circular curved portion 37a of the first dividing line 37. Therefore, the second and third regions 32 and 33 are formed in a circular-ring-shaped one-quarter circle.
As the hologram 30 having such a hologram pattern is used, the reflected light from the other recording layer, of which the beam size is small, enters the axis neighboring portion 36 and can be prevented from entering the second and third regions 32 and 33 for generating a TES. Therefore, it is possible to obtain accurate tracking position information and position deviation information.
However, the hologram pattern proposed in JP-A 2004-303296 has the following problem. When information is recorded and reproduced onto and from the first recording layer of the disk having two recording layers, a part of light transmits through the first recording layer into the second recording layer and is reflected therein. At this time, light diffracted by a land/group, which is formed in the second recording layer, as well as the reflected light is generated.
FIG. 12 is a view showing a state where the diffracted light diffracted by the second recording layer enters the hologram element 30. The diffracted light diffracted by the second recording layer forms a first spot 41, a second spot 42 and a third spot 43. The first spot 41 falls on the first dividing line 37 across the first and second regions 31 and 32 on the hologram pattern. The second spot 42 falls on the first dividing line 37 across the first and second regions 31 and 33 on the hologram pattern. The third spot 43 is zero-order diffracted light which falls on the axis neighboring portion 36. The first and second spots 41 and 42 respectively enter the second and third regions 32 and 33 serving as the splitting section for tracking which splits light for TES generation. Therefore, the light entering the second and third regions 32 and 33 are respectively diffracted so as to be introduced into the second and third light receiving sections 7B and 7C for TES detection.
FIG. 13 is a diagram showing a state where the diffracted light by the second recording layer enters the light receiving unit 7. The diffracted light spots 41 and 42, which are diffracted by the second recording layer so as to enter the hologram element 30, respectively form greatly-extended spots 41a and 42a on the second and third light receiving sections 7B and 7C. Further, the diffracted light spots 41 and 42 also enter the fourth and fifth light receiving elements 7g and 7h and the seventh and eighth light receiving elements 7e and 7f serving as light receiving elements for sub-beams for obtaining position signals.
The light intensity of the sub-beam is about one tenth of that of the main beam. Therefore, the amplification degree with respect to detection output by the light receiving element receiving the sub-beam is set to be larger than the amplification degree with respect to detection output by the light receiving element receiving the main beam. Therefore, the diffracted light (stray light) by the second recording layer has a large effect on the sub-beam of the original reflected light from the first recording layer. Further, noise components and offset components, which are not necessary for lens position signal and TES, are generated by the stray light, thereby degrading tracking servo characteristics.
In other words, the hologram pattern of the hologram element 30 proposed in the JP-A 2004-303296 can exclude an effect of the simple reflected light (zero-order diffracted light) by the second recording layer. However, the hologram pattern cannot exclude an effect of the diffracted light by the land/group of the second recording layer.